Novel bonding structure for a hard disk drive suspension using anisotropic conductive film

ABSTRACT

A plurality of bonding structures and their forming methods for bonding a FPC to a bonding pad, in particular a bonding pad of a wireless suspension in a head gimbal assembly, using anisotropic conductive adhesive; such structures eliminate the spring-back force in typical anistropic bonding to ensure durable bonding. At the same time, these structures also allow for reworkability under which the bonded parts can be separated easily.

BACKGROUND OF THE INVENTION

[0001] This invention generally relates to the field of disk drives, andmore particularly to forming optimal structures for bonding in a headgimbal assembly using anisotropic conductive adhesive.

[0002] With the rapid progress of miniaturizing and thinning technologyfor electronic devices, high-density inner wiring systems includingflex-print circuit (FPC) have become essential. At the same time,micro-connecting technology for the connection of FPC with otherelectronic parts, such as the traces on a magnetic head suspensionassembly, is indispensable.

[0003] Traditionally the FPC is capable of adopting ultrasonic bonding.The connecting terminals of the FPC are plated with gold; the flyingleads of the FPC are aligned with and pressed to the bonding pad on thesuspension with sufficient force to keep the alignment and atomicinterdiffusion of the flying leads and the underlying metallization,which process ensures the intimate contact between the two metalsurfaces. However, the pressing of the flying leads of the FPC entailscomplex processing, and ultrasonic bonding to different bonding pads isvery difficult to contact Moreover, bonded parts cannot be separated inthe future to be reworked without damaging the FPC or the suspension.

[0004] Alternatively, FPC can be solder-bound using solder bumpsproduced by, for example, plating processes, for interconnections.However, this process requires forming metal cores and solder bumps forsoldering. The metal cores incur extra expenses, and soldering has to beperformed at high temperatures typically over 180 degrees Celsius.

[0005] Furthermore, both ultrasonic bonding and soldering are becomingincreasingly expensive because of high cost of labor and parts of theFPC. There is therefore a need for a bonding method which achieves astable, reworkable connection without complicated processing.

SUMMARY OF THE INVENTION

[0006] The present invention features a novel structure and method forusing anisotropic conductive adhesive to bond parts in a head gimbalassembly (HGA) comprising the slider and the FPC.

[0007] It is an object of the present invention to overcome thecomplexities of prior art approaches of ultrasonic bonding andsoldering. This invention will alleviate the difficulty of one-timebonding in the case of ultrasonic bonding, and avoid high-temperaturebonding required in soldering.

[0008] It is another objective of the present invention to reduce thebonding pad size and floating capacity.

[0009] Yet another objective of the present invention is to reduce thespace between bonding pads to accommodate the trend towardminiaturization of the disk drives and the head assemblies.

[0010] A further related objective of the invention is to improvecapacity in the bonding process. Reduced sizes of the bonding pads,reduced spacing between the bonding pads, and elimination of additionalinterconnecting components will contribute to reduce parasiticcapacitance. Reduced capacitance will improve the rise and fall time ofthe electronic signals, thus increase the data rate of the hard diskdrive.

[0011] In one aspect, the invention relates to adding a conductingstructure lodged between the two sections of an overcoat layer of a FPCto enable bonding between the FPC and a contact pad in a HGA usinganisotropic conductive adhesive, such as anisotroppic conductive film(ACF). The conductive structure can be shaped as a ball and plated withgold, or it can of other types of conductive materials. The overcoatlayer may overlap a portion of the top surface of the conductive pad, orthe overcoat layer may not touch the conductive pad at all.Alternatively, the conductive structure may be a filler comprising anelectrically conductive material completely filling the space betweenthe two sections of the overcoat layer and above the conductive pad. Inone implementation, the overcoat layer may comprise one section, or itmay be of ultra thinness of less than 10 μm.

[0012] In another aspect of the invention, a conductive layer of the FPCmay be bound to the contact pad directly by anisotropic conductiveadhesive material without an overcoat layer in between.

[0013] Other features and advantages of the present invention willbecome apparent from the following drawings and the detailed descriptionaccompanying the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a top view of a wireless suspension of a head gimbalassembly.

[0015]FIG. 2 is a top view of a FPC bound to the wireless suspension ofFIG. 1.

[0016]FIG. 3 is cross-sectional view of the structure of a conventionalFPC.

[0017]FIG. 4 is a cross-sectional view of the structure of a wirelesssuspension bonding pad.

[0018]FIG. 5A is a cross-sectional view of the conventional FPC of FIG.2 positioned on top of the wireless suspension bonding pad of FIG. 4.

[0019]FIG. 5B is a cross-sectional view of the conventional FPC of FIG.2 bound to the wireless suspension bonding pad of FIG. 4 usinganisotropic conductive adhesive.

[0020]FIG. 5C is a cross-sectional view, after reliability test, of aconventional FPC of FIG. 2 bound to the wireless suspension bonding padof FIG. 4 using anisotropic conductive adhesive.

[0021]FIG. 6 is a cross-sectional view of a novel bonding structurebetween a FPC and a wireless suspension using anisotropic conductiveadhesive.

[0022]FIG. 7 is a cross-sectional view of a second novel bondingstructure of a FPC.

[0023]FIG. 8 is a cross-sectional view of a third novel bondingstructure between a FPC and a wireless suspension using anisotropicconductive adhesive.

[0024]FIG. 9 is a cross-sectional view of a fourth novel bondingstructure between a FPC and a wireless suspension using anisotropicconductive adhesive.

[0025]FIG. 10 is a cross-sectional view of a fifth novel bondingstructure of a FPC.

[0026] Like parts in different drawings are labeled with like numbers

DESCRIPTION OF THE PREFERED EMBODIMENT

[0027] Referring to FIG. 1, this is a standard wireless suspension.Trace 112 is patterned on top of a flexture piece which runs from slider120 to bonding pads 102, 104, 106, and 108, transporting electromagneticsignals from slider 120. Base plate 100 supports bonding pads 102, 104,106, 108, to which a FPC is bonded for transmitting signals to elsewherein a hard disk drive, such as a circuit on the actuator arm. The numberof contact pads shown here is for illustrative purposes only, and therecould be more or fewer contact pads without deviating from the spirit ofthe invention.

[0028] Referring to FIG. 2, a FPC 200 is attached to contact pads 102,104, 106, 108 (not shown) in the circled area 210. Traditionally, FPCcan be bound to contact pads using ultrasonic bonding or soldering. Withsoldering, additional solder bumps need to be incorporated As mentioned,both prior art bonding methods tend to be cost- and labor-intensive, andbonding using anisotropic conductive adhesive, such as anisotropicconductive film (ACF) CP9252KS by Sony Corporation of Tokyo, Japan,presents a good alternative.

[0029] ACF bonding requires bonding temperature of 150 to 200 Celsius,and a pressure environment of 20 to 40 kg per square centimeters. Thebonding time is about 10 to 20 seconds. The process involves cutting theACF into pieces of desirable size, tacking the pieces unto the surfaceto be bound, removing the release liner, and bonding under theconditions set out above. ACF bonding also offers the advantage ofreworkability. For example, Sony CP9252KS can be reworked by dipping itin acetone for 2 minutes, peeling the ACF, and following up with a Q-tiptouch with acetone. ACF bonding also offers good bonding strength. Forexample, ultrasonic bonding typically offers a bonding strength of about60 g, comparing with more 130 g for ACF bonding.

[0030] Despite the advantages offered by ACF bonding, difficultiesremain for applying ACF bonding to a head gimbal assembly. For example,FIG. 3 shows a cross-sectional view of a conventional FPC structure. Aconventional FPC 200 usually comprises a base film 301, two sections 305and 309 of an overcoat layer, with an in-between conductive layer 303between base film 301 and the overcoat layer. Base film 302 is usuallymade of insulation material such as polyimide or other types of resinThe sections 305 and 309 of the overcoat layer is made of solder epoxy,photo sensitive solder resist materials, or polyimide film. Theconductive layer 303 is usually made of Cu or other similar materials.Between the sections 305 and 309 is the bonding pad surface 307, usuallywith a plating of Ni with thickness of about 4 μm and a plating of Auwith thickness of 1 μm.

[0031]FIG. 4 illustrates cross-sectional view of an assembly 400comprising a wireless suspension bonding pad, such as bonding pad 108 ofFIG. 1. Assembly 400 comprises stainless steel base 401, on top of whichis an insulating layer 403. Insulating layer 403 can be made ofpolyimide or other types of insulating resin. Bonding pad 108 ispositioned on top of layer 403, and it comprises, in a typicalconfiguration, an electrode 405 made of Cu, followed by a plating 407 ofNi, and finally a plating 409 of gold at the outermost surface ofbonding pad 108.

[0032] FIGS. 5A-5C illustrate some of the problems of using ACF to bondthe FPC 200 to the assembly 400. FIG. 5A shows that the FPC 200 ispositioned on top of assembly 400, with bottom surfaces of sections 305and 309 overlapping the two ends of bonding pad 108. When ACF film isheated and applied to bond the two components using bonding tools andprocessing conditions as set forth above, a deformation 510 in the shapeof a bridge is formed to make contact between the FPC 200 and assembly400, as shown in FIG. 5B. Unfortunately, after reliability test, thisdeformation 510 tends to revert back to its original condition, causingan open circuit problem, as shown in FIG. 5C. Therefore, several novelbonding structures have been invented to solve this open circuitproblem.

[0033] Illustrated in FIG. 6 is a ball structure 610 which is placedbetween the conductive layer 303 and the top surface of bonding pad 108.The ball structure 610 can be made of gold in one implementation, or itcan be made of other materials in other implementations of the inventionThe ball structure 610 can be formed, in one implementation, with studbump bonding (SBB) flip chip method or gold ball bonding method commonlyknown in the art. The space surrounding ball structure 610, as well asspace 605 and 607, will be filled with melted/cured ACF used forbonding. The presence of structure 610 prevents the deformation of theFPC, and therefore eliminates the open circuit problem. Typically, for abase film of thickness 23 μm, the conductive layer is about 18 μm, andthe overcoat layer about 13 μm. Therefore, the ball structure, or bump610, has a height of approximately 13 μm. Circuit traces are labeled as601 and 602 in FIG. 6.

[0034] Alternatively, as illustrated in FIG. 7, the complete spaceformed by the top surface of bonding pad 108 (not shown), the bottomsurface of conductive layer 303, and the right wall of overcoat section305 and overcoat section 309 can be filled with filling materials 700.The thickness of this filling 700 is about 13 μm, and it be made of anumber of conductive materials including Ni, Au, or a combinationthereof. In other implementations of the invention, the filling 700 canbe thicker, thinner, to equal to the thickness of the overcoat layer,ranging between 10 to 38 μm. Using a solid filling 700 will achieve thesame objective of eliminating the deformation bridge 510, and therebypreventing the open circuit problem. Note that adhesive layers used inthe manufacturing process of FPC 200 may still be present between thebase film 301 and conductive layer 303, and/or between conductive layer303 and overcoat sections 305 and 309.

[0035] Another implementation of the invention is the removal of one ofthe two overcoat sections. In this configuration, as illustrated in FIG.8, ball structure 610 is still present, but the remaining section 805,the conductive layer 803 and the base film 801 are all of shorter lengththan their counterparts in a FIG. 6. This approach reduces the amount ofmanufacturing materials required. Melted/cured ACF fills spacesurrounding ball structure 610 and space 810.

[0036]FIG. 9 illustrates yet another implementation of the invention. Inthis configuration, only one of the two sections of overcoat layer ispresent. The bottom surface of section 905 does not overlap the topsurface of bonding pad 108. Furthermore, this configuration does notrequire ball structure 610. At the same time conductive layer 903 bindsto the top surface of bonding pad 108 directly using ACF bonding, butdoes not overlap the top surface completely. Base film 901 extendsbeyond the length of bonding pad 108, but stops before reaching circuittrace 602. Eliminating the overcoat layer in a FPC will minimize theopen circuit problem; however, overcoat section 905 is needed to preventthe shunting problem around the complicated circuit pattern around thebonding pad. This contrasts with the right hand side of bonding pad 108,where conductive layer 903 does not touch trace 602 because of theabsence of an overcoat layer between it and trace 602. Therefore, thisconfiguration presents an optimal compromise between the elimination ofthe bridge deformation in a FPC inherent in ACF bonding, and theprevention of shunting problem around a bonding pad's complicatedcircuitry.

[0037]FIG. 10 illustrates another novel structure of FPC using ACFbonding. Because, as mentioned above, that it is impossible to eliminatethe overcoat layer completely, one solution is to form an ultrathinovercoat layer, such as presented in FIG. 10. Overcoat sections 1005 and1010 are of less than 10 μm thick. They are think enough to prevent theshunting problem, but thin enough to prevent the formation of adeformation bridge in ACF bonding. Because sections 1005 and 1010 arethin, bonding surface 1000 can bond directly to the top surface of abonding pad without causing a deformation in base film 301 andconductive layer 303.

[0038] The above embodiments of the invention are for illustrativepurposes only. Many widely different embodiments of the presentinvention may be adopted without departing from the spirit and scope ofthe invention. Those skilled in the art will recognize that the methodand structures of the present invention has many applications, and thatthe present invention is not limited to the specific embodimentsdescribed in the specification and should cover conventionally knownvariations and modifications to the system components described herein.

What is claimed is:
 1. A flex-print circuit (FPC) attached to at leastone bonding pad on a suspension of a head gimbal assembly in a hard diskdrive using anisotropic conductive adhesive, comprising: a base film; aconductive layer situated below the base film; an overcoat layercomprising at least two sections situated below the conductive layer, abottom surface of each section overlapping partially a top surface ofthe bonding pad; and a conductive structure forming an electric conduitbetween the conductive layer and the at least one bonding pad, saidanisotropic conductive adhesive being disposed at least surrounding theconductive structure for bonding the FPC to the at least one bondingpad.
 2. The FPC of claim 1, wherein the conductive structure comprisesgold.
 3. The FPC of claim 1, wherein the anisotropic conductive adhesivecomprises anisotropic conductive film.
 4. The FPC of claim 1, whereinthe conductive structure comprises a bump having a height of about 12 to38 μm.
 5. The FPC of claim 1, wherein the conductive structure comprisesa gold ball.
 6. The FPC of claim 1, wherein the conductive structurecomprises a filling completely occupying a space formed by the at leastone bonding pad, the conductive layer, and the at least two sections ofthe overcoat layer.
 7. The FPC of claim 6, wherein the filling is lessthan 10 μm thick.
 8. The FPC of claim 6, wherein the filling is about 10to 38 μm thick.
 9. The FPC of claim 6, wherein the filling is thickerthan or equal to the overcoat layer.
 10. The FPC of claim 6, wherein thefilling is thinner than the overcoat layer.
 11. A head gimbal assembly(HGA) circuit structure attached to a bonding pad on a suspension of ahead gimbal assembly for use in a hard disk drive using anisotropicconductive adhesive, comprising: a base film; a conductive layersituated below the base film, a part of said conductive layer attachedto the bonding pad using said anisotropic conductive adhesive; and anovercoat layer situated below a portion of the conductive layer, abottom surface of said overcoat layer not overlapping a top surface ofthe bonding pad.
 12. The HGA circuit structure of claim 11, furthercomprising a conductive ball positioned above the bonding pad forming anelectric conduit between the conductive layer and the bonding pad. 13.The HGA circuit structure of claims 12, wherein the conductive ballcomprises gold.
 14. The HGA circuit structure of claim 11, wherein theanisotropic conductive adhesive comprises anistropic conductive film.15. The HGA circuit structure of claim 11, wherein a portion of saidconductive layer is bonded to the top surface of the bonding paddirectly using said anisotropic conductive adhesive.
 16. A method forbonding a flex-print circuit to a suspension in a head gimbal assembly,comprising the steps of: Forming a conductive structure between abonding pad and a conductive layer of the flex-print circuit; andBonding the conductive layer to the bonding pad via the conductivestructure using anisotropic conductive adhesive.
 17. The method of claim16, wherein the anisotropic conductive adhesive comprises anisotropicconductive film.
 18. The method of claim 16, wherein the conductivestructure comprises a gold ball.
 19. The method of claim 16, wherein theconductive structure comprises a solid conductive material filling. 20.The method of claim 18, wherein the gold ball is formed using stud bumpbonding (SBB).
 21. A flex-print circuit (FPC) attached to a bonding pad,comprising A conductive layer bonded to the bonding pad usinganisotropic conductive adhesive; and A conductive bump lodged betweenthe conductive layer and the bonding pad.
 22. The FPC of claim 21,wherein the conductive bump comprises gold.
 23. The FPC of claim 21,further comprising an overcoat layer positioned below the conductivelayer.
 24. The FPC of claim 23, wherein the overcoat layer comprises twosections separated by a plating of conductive material, each of said twosections overlapping an end of a top surface of the bonding pad.
 25. TheFPC of claim 23, wherein the overcoat layer does not overlap the bondingpad.
 26. The FPC of claim 21, wherein the anisotropic conductiveadhesive comprises anisotropic conductive film